Is there a way to get the exterior surface of a complex model in Blender?
There are at least three ways to do this. One is to select the whole object, and apply the decimate modifier; another is to create a pair of planes, one above and one below, and apply to the planes a shrinkwrap modifier. The third method is to use the retopology idea offered by @poor. However, while any of these might yield suitable results, they each require a certain level of mastery of Blender to be able to do. In my view, you'd be well served to spend some time mastering Blender, before you attempt what you are trying to do.